Salt bath nitriding treatment is widely used to improve material properties such as abrasion resistance and fatigue strength of metals, especially iron and steel, by forming both nitrided layers and nitrogen diffusion layers on their surfaces. This salt nitriding treatment is applied not only to plain steel but also to alloy steel such as stainless steel and nickel-based alloys (so-called super alloys) represented by “Inconel” and the like.
Such nitrided layer and nitrogen diffusion layer, which have been obtained by the above-described method, have a function to heighten the surface hardness of the associated metal member such that the metal member is improved in abrasion resistance and fatigue strength and at the same time, is protected from a corrosive loss. Conventional salt bath nitriding treatment, therefore, needs no further treatment insofar as corrosion resistance of an ordinary level is required. Further treatment is, however, needed in an applications where corrosion resistance is required to such an extent as available from hard chromium plating as one of competing surface hardening techniques. To make improvements in the corrosion resistance of metal members nitrided as described, a variety of inventions have been made (see, JP 56-33473 A, JP 60-211062 A, JP 05-263214 A, JP 05-195194 A, JP 07-62522 A, and JP 07-224388 A).
To make further improvements in corrosion resistance, combinations of nitriding treatment and oxidizing bath treatment have also been proposed (see, JP 56-33473 A and JP 07-224388 A). Corrosion resistance available from such combined treatment is stated to be comparable or better compared with hard chromium plating when determined by the salt spraying test. However, the corrosion resistance available from such combinations of salt bath nitriding treatment and oxidizing bath treatment varies so much that their adoption has been avoided in many instances. With a view to overcoming this drawback, it has also been proposed to impregnate a treated product with wax or to coat a treated product with a polymer after the application of the nitriding treatment and oxidizing bath treatment in combination (see, JP 05-195194 A and JP 05-263214 A).
These two methods are intended to achieve both an improvement and stabilization (improved reproducibility) in corrosion resistance by conducting the wax impregnation or polymer coating treatment such that the coefficient of friction is lowered to make an improvement in abrasion resistance and at the same time, an oxide layer is sealed or covered with a wax or polymer coating. These two methods can bring about good material properties such as high abrasion resistance and fatigue strength and at the same time, improvements in corrosion resistance and its reproducibility.
Nonetheless, the incorporation of an impregnation or coating step in addition to oxidizing bath treatment after a nitriding step is not readily acceptable in view of factors such as initial cost, productivity, production cost and the like.
The present inventors, therefore, invented a method of forming an oxide layer, which is excellent in barrier properties, on an outermost surface concurrently with achieving nitriding upon subjecting a metal member, especially an iron-based member to nitriding treatment in a salt bath, and succeeded in imparting corrosion resistance, which is superior to that available from hard chromium plating, in addition to making improvements in material properties such as abrasion resistance and fatigue strength. An application for a patent was filed on the invention (Japanese Patent Application No. 2001-361544, now JP 2002-226963 A).
The above-described method features that upon forming a nitrided layer on a surface of a metal member, especially an iron-based member by immersing the metal member in a molten salt bath containing Li+, Na+ and K+ ions as cation components and CNO− and CO3−− ions as anion components, the oxidizing power of the salt bath is enhanced by addition of an alkali metal hydroxide, bound water, free water, moist air or the like to form, concurrently with formation of a nitrided layer on a surface of the member, an oxide layer on an outermost surface of the nitrided layer.
The oxide layer is a thin layer composed of a lithium iron oxide layer and having a thickness as small as 0.5 to 5 μm, but is equipped with an excellent barrier function against chlorine ions as a corrosive environment factor and can significantly improve the corrosion resistance of a nitrided metal member. The method disclosed in JP 2002-226963 A is, therefore, expected to find wide-spread utility as a surface hardening method capable of imparting high corrosion resistance as a substitute method for hard chromium plating.
With respect to stainless steel widely employed as a corrosive metal material, salt bath nitriding, ionitriding, gas nitriding and the like are also practiced for applications each of which requires an improvement in surface hardness. These nitriding treatment methods are, however, accompanied by a drawback that a passivated film on a surface of stainless steel is destroyed to impair the corrosion resistance which stainless steel is inherently equipped with (see JP 2001-214256 A) Therefore, the hard chromium plating has been applied for the improvement of surface hardness of stainless steel with inherent corrosion resistance, although the plating film has problems of unsatisfactory adhesion and the like.
The method disclosed in JP 2002-226963 A can form, concurrently with nitriding a surface of stainless steel, a lithium iron chromium oxide layer having good adhesion and high corrosion resistance on an outermost surface. This method is, therefore, expected to find practical utility as a surface hardening method for stainless steel as a substitute method for hard chromium plating.
Reference is next had to FIGS. 1A through 2B. FIGS. 1A and 2A are cross-sectional schematics of surface-modifying layers formed on plain steel and stainless steel, respectively, by a conventional method, while FIGS. 1B and 2B are cross-sectional schematics of surface-modifying layers formed on plain steel and stainless steel, respectively, by the method disclosed in JP 2002-226963 A. In these drawings, there are shown nitrogen diffusion layers 1 (thickness: 0.2 to 1 mm), compound layers 2 (also called “white layers”, Fe2N, thickness: 5 to 30 μm), a black lithium iron oxide layer 4 (thickness: 0.5 to 5 μm), nitrogen diffusion layers 11 (thickness: 0.2 to 1 mm), first compound layers 12 (also called “white layers”, Fe2N+Cr2N, thickness: 10 μm), second compound layers 13 (also called “black layers”, CrN+Fe2N, thickness: 20 to 80 μm), and a black lithium iron chromium oxide layer 14 (thickness: 0.5 to 5 μm) The lithium iron oxide layer 4 and lithium iron chromium oxide layer 14, both of which have been formed by the method disclosed in JP 2002-226963 A, are extremely thin layers, but are excellent in barrier effects against chlorine ions and the like as corrosive environment factors and contribute to improvements in the corrosion resistance of the nitrided materials. On the other hand, the compound layers 2, 12, 13 shown in the drawings have high hardness and impart superb abrasion resistance to the plain steel and stainless steel. The nitrogen diffusion layers 1 and 11 formed below the compound layers 2 and 12, respectively, are solid solution layers with nitrogen dissolved in the plain steel and stainless steel, respectively. Owing to the compression stress produced as a result of dissolution of nitrogen, the resulting members are provided with significantly-improved fatigue strength.
To obtain such a nitrogen diffusion layer, it is necessary to quench a member from a temperature of at least 300° C. or higher subsequent to its nitriding treatment. In salt bath nitriding by the method disclosed in JP 2002-226963 A, quenching is also conducted at 450 to 650° C. as in conventional salt bath nitriding treatment. Taking into consideration residual strain in the treated product, prohibition of γ′ (Fe4N) deposition in a nitrogen diffusion layer, and the like, however, post-nitriding quenching is conducted by one of the following three methods, said one quenching method being selected to obtain target material properties:                Salt bath nitriding→water quenching→hot water rinsing→drying.        Salt bath nitriding→oil quenching→hot water rinsing→drying.        Salt bath nitriding→air quenching→hot water rinsing→drying.        
Water quenching is the highest in quenching rate, and is adopted when importance is placed on the inhibition of γ′ (Fe4N) deposition in a nitrogen diffusion layer. Air quenching, on the other hand, is the lowest in quenching rate and is adopted when importance is placed on the inhibition of residual strain. Oil quenching is selected in view of a balance between quenching rate and strain. To achieve both of the prevention of residual strain and the inhibition of γ′ (Fe4N) deposition, air quenching may be applied to around 400° C., following by water quenching.
As one example of the compositions of conventional molten salt nitriding baths, the following composition can be mentioned: 35 wt. % CNO−, 18 wt. % CO3−−, 3.5 wt. % Li+, 18 wt. % Na+, 22.5 wt. % K+, and 3 wt. % CN− (hereinafter called “the salt bath C”). As an illustrative composition of a molten salt nitriding bath for use in the method disclosed in JP 2002-226963 A, on the other hand, the following composition can be mentioned: 15 wt. % CNO−, 40 wt. % CO3−−, 4 wt. % Li+, 18 wt. % Na+, 22.5 wt. % K+, and 0.5 wt. % CN− (hereinafter called “the salt bath N”).
To permit formation of an oxide layer on an outermost layer concurrently with nitriding, the salt bath for use in the method disclosed in JP 2002-226963 A has such a formula design that contains CNO−, a source component for the formation of cyanide, at a minimized level to reduce CN−, which is a reducing substance and has dissolving action on iron oxides, to as low a concentration as possible. As a result, the proportion of a carbonate having a relatively low solubility in water is greater compared with the corresponding proportion in the conventional bath.
Subsequent to the salt bath nitriding, the treated product is subjected to water quenching (or oil quenching or air quenching) to quench it, and is then rinsed with hot water in the subsequent step. As the conventional salt bath contains a cyanate, which has high solubility in water, in a large proportion, the molten salt adhered on the treated product can be readily dissolved and rinsed off with water. In the salt bath for use in the method disclosed in JP 2002-226963 A, on the other hand, the carbonate which is lower in solubility than the cyanate is contained in a large proportion. The molten salt dragged out in a state adhered on the treated product, therefore, tends to remain on the treated product without being completely rinsed off with water where the treated product is a part of complex configurations, although such a molten salt can be readily rinsed off with water in the case of a part of simple configurations. In general, no molten salt is allowed to adhere and remain on a treated product. Especially in the case of a molten salt nitriding bath in which by produced cyanides exist although they are contained only in trace amounts, the molten salt is by no means allowed to remain on the treated product.
In the salt bath composition for use in the method disclosed in JP 2002-226963 A, the reduction in the content of the cyanate is replaced by the carbonate for the reasons to be mentioned next. The nitriding of steel in a salt bath is known to take place by solid diffusion of nascent nitrogen, which is produced by decomposition of a cyanate by the following formula (1) or (2), into the steel:4MeCNO→2MeCN+Me2CO3+CO+2N  (1)5MeCNO→3MeCN+Me2CO3+CO2+2N  (2)wherein Me represents a monovalent alkali metal.
The cyanide formed by the reaction of the formula (1) or (2) is considered to be an effective component, because it is oxidized and converted back into the effective cyanate through the following reaction by salt bath aeration conducted as a standard procedure upon performing salt bath nitriding:2MeCN+O2→2MeCNO  (3)
The carbonate formed by the reaction of the formula (1) or (2), on the other hand, accumulates as the salt bath nitriding treatment proceeds. Before the technique disclosed in JP 51-5024A was invented, cyanate the content of which dropped through the treatment was replenished with an alkali metal cyanide. Due to accumulation of the unnecessary carbonate, however, the replenishment of a fresh supply of the alkali metal cyanate was hardly feasible unless a portion of the salt bath was discarded. The invention disclosed in JP 51-50241 A made it possible to maintain the concentration of a cyanate in the salt bath without pumping out the old salt, which contains a toxic cyanide, by reacting a useless carbonate, which is contained in the salt bath, with a nitrogen-containing organic compound to convert it back directly into the effective cyanate.
The conversion back into the cyanate when urea is used as a nitrogen-containing compound can be represented by the following formula:Me2CO3+2CO(NH2)2→2MeCNO+2NH3+CO2+H2O  (4)
The above description is believed to make it possible to understand the inevitability of the salt bath composition of MeCN/MeCNO/Me2CO3, that is, the reason for the replacement of the reduction in the content of MeCNO with Me2CO3.